Functional characterization of xanthoxin dehydrogenase in rice

Abscisic acid (ABA) is a phytohormone that plays a key role in biotic and abiotic stress responses. ABA metabolic genes are promising targets for molecular breeding work to improve stress tolerance in crops. The accumulation of ABA does not always improve stress tolerance since stress-induced accumulation of ABA in pollen inhibits the normal course of gametogenesis, affecting grain yields in cereals. This effect highlights the importance of manipulating the ABA levels according to the type of tissues. The aim of this study was to assign an ABA biosynthetic enzyme, xanthoxin dehydrogenase (XanDH), as a functional marker to modulate ABA levels in rice. XanDH is a member of the short-chain dehydrogenase/reductase family that catalyzes the conversion of xanthoxin to abscisyl aldehyde (ABAld). Previously, this enzyme had only been identified in Arabidopsis, as AtABA2. In this study, a XanDH named OsABA2 was identified in rice. Phylogenetic analysis indicated that a single gene encodes for OsABA2 in the rice genome. Its amino acid sequence contains two motifs that are essential for cofactor binding and catalytic activity. Expression analysis of OsABA2 mRNA showed that the transcript level did not change in response to treatment with ABA or dehydration. Recombinant OsABA2 protein expressed in Escherichia coli converted xanthoxin to ABAld in an NAD-dependent manner. Moreover, expression of OsABA2 in an Arabidopsis aba2 mutant rescued the aba2 mutant phenotypes, characterized by reduced growth, increased water loss, and germination in the presence of paclobutrazol, a gibberellin biosynthesis inhibitor or high concentration of glucose. These results indicate that OsABA2 is a rice XanDH that functions in ABA biosynthesis.